CCDs for directly detecting X-rays are now widely used, particularly in the field of astronomy (see G. W. Frazer: "X-Ray Detectors in Astronomy," Cambridge University press (1989) 208; and B. E. Burke, R. W. Mountain, P. J. Deniels, M. J. Cooper, and V. S. Dolat: IEEE Trans. Nuclear Sci. 41 (1994) 375).
A CCD is an integral-type sensor which can increase the resolution of an image to a level corresponding to pixel size. If the flux of incident X-rays can be reduced to such a level that primary electron clouds produced by X-ray photons do not overlap each other, in addition to an X-ray image, an X-ray energy spectrum can be obtained.
At present, a CCD is widely used as an X-ray photon counter, particularly in the field of astronomy. When the flux of incident X-rays is reduced to such a level that primary electron clouds produced by X-ray photons do not overlap each other, in addition to an X-ray image, an X-ray energy spectrum can be obtained.
Recently, the present inventors proposed a new technique, by which the X-ray response of a CCD can be obtained with subpixel resolution (Japanese Patent Application No. 9-82444). The new technique employs a metal mesh placed in proximity to a CCD, and a parallel X-ray beam. The mesh has a large number of small holes with periodic spacing, the spacing being equal to the CCD pixel size. Hereinafter, such a mesh is referred to as a "single-pitch mesh." Since the position of an input X-ray can be determined with subpixel resolution, X-ray response can be investigated in detail.
FIG. 1 shows the overall structure of an apparatus for measuring X-rays detected by use of a CCD. FIG. 2 is a schematic view showing an experiment performed by use of a single-pitch mesh.
In FIGS. 1 and 2, reference numeral 1 denotes an X-ray generator, 2 denotes a CCD camera, 3 denotes a CCD, 4 denotes a single-pitch metal mesh, and 4a denotes holes of the single-pitch metal mesh.
As shown in FIG. 2, the metal mesh 4 having periodically spaced holes is placed in close proximity to the surface of the CCD 3. The mesh 4 is placed parallel to the surface of the CCD 3 and tilted by a small angle .theta. with respect the pixel axis. A parallel X-ray beam is radiated normal to the surface of the CCD 3. The X-rays passing through the mesh holes 4a reach the CCD 3. The position of X-ray interaction (hereinafter referred to as an "X-ray interaction position") is restricted by the mesh hole 4a. An X-ray event whose interaction position is far from a pixel boundary should take place in a pixel where a mesh hole 4a is located at the center thereof. In such a situation, a single-pixel event usually occurs.
Meanwhile, an X-ray event whose interaction position is close to a pixel boundary takes place in a pixel which has more than two hole shadows. In this case, a split event occurs. When a single-pitch mesh is used, the single-pixel event and the split event cannot be distinguished from each other, because the mesh holes 4a have a fixed size, and spacing between holes is equal to the inter-pixel distance. Therefore, even when a pixel at which an X-ray event occurs can be determined, the hole through which the X-ray enters cannot be determined.